An electro-hydraulic control system controls shifting and operation of automatic vehicle transmissions. To permit shifting by the vehicle operator, the electro-hydraulic control system typically includes either a manual valve or a shift by wire valve.
In a manual valve electro-hydraulic control system, the vehicle operator manually changes the position of the valve to accomplish certain shifts, for example, to initiate movement of the vehicle from a non-moving state to a moving state or vice versa, or to change the direction in which the vehicle is moving (i.e., shifts from neutral to a forward range, from neutral to a reverse range, from a reverse range to a forward range, from a reverse range to neutral, from a forward range to neutral, or from a forward range to a reverse range).
In a shift by wire control system, at least some of the inputs that are used to initiate shifting are in the form of electrical signals rather than hydraulic or mechanical forces. Instead of a manual shift selector, a push button range selector may be used in a shift by wire system. Activation of a push button or similar actuator by the vehicle operator sends an electrical signal to an electronic control unit. The electronic control unit executes computer logic to determine which valve(s) in the electro-hydraulic control system need to change position in order for pressurized hydraulic fluid to be directed to the appropriate clutches to accomplish the requested shift. The electronic control unit sends electrical signals to solenoid valves of the electro-hydraulic control system, which initiate the valve position changes required to accomplish the requested shift.